Cotton: burndown and rut repair

It’s almost time to start thinking about burndown for the 2010 season and we are not completely out of the 2009 fields — we still have some stalks and a bunch of ruts to contend with.

But the new Cardui calendar indicates the holiday and hunting seasons are behind us and another year is in the making.

First, let’s address the fields with ruts. A disk will be the tool of necessity in many fields where pickers and combines were forced into situations last fall we didn’t want, but had no other choices. If the weather breaks and these fields are worked more than 45 days prior to planting, it may be advantageous to consider a soil residual herbicide after working to keep them from growing up before planting.

This is especially true in areas where horseweed is likely to be a problem. We used to think that horseweed would not come in behind a winter or spring disk, but this is no longer true. We have noted this in our efforts to control horseweed, but recent research shows the glyphosate-resistant horseweed biotype germinates over a longer period of time than its susceptible cousins.

I have gotten calls over the past two years about controlling horseweed that has germinated after cotton has emerged. Valor is probably the residual herbicide most commonly used in burndown programs, but others such as Direx and Caparol work well also. You must know what you plan to plant in the field and use a residual that is approved for that crop.

Valor has a 30-day plant-back to cotton and has no restrictions to soybean. If beds are to be re-run or a harrow used before planting, a residual herbicide may not be necessary. However, remember that bedders usually transplant horseweed up on the bed rather than destroy it — especially if the weed is greater than 2 inches tall.

At one time, I was fairly critical of anyone spraying weeds if he planned to rework the beds prior to planting. If horseweed is present, I say spray it at least 10 days prior to working the beds. This gives the herbicide time to translocate in the weed and dissipate from the soil surface and not be buried in the bed.

Fields that were worked in the fall will likely be haired over this spring. It seems that horseweed drives most of our burndown decisions. We worry about horseweed control, then add something to control other weeds. Some have resorted to a split burndown program with one application made in mid-February and another 21 days prior to planting. No doubt this double shot is effective, but also expensive.

My standard recommendation for controlling horseweed is 8 ounces of a dicamba product like Clarity tank-mixed with glyphosate or other herbicide to control other weeds present. I usually add a residual herbicide to hold the horseweed until planting. We have sprayed over 1,000 plots looking for the magic horseweed material and have found nothing more effective than dicamba.

Unfortunately, neither glyphosate nor dicamba is extremely good on henbit. Adding a pint of 2,4-D or 0.3 ounce of Harmony Extra XP will improve henbit control. I usually feel a good burndown can be accomplished in one application; however, we did have some failures a couple years ago when the weather was extremely dry and cold all spring. The weeds were not growing and failed to take in the herbicide and the extremely dry soil did not allow any soil activity from dicamba. The weeds curled and continued living.

Most of our problems with burndown stem from improper application. I seldom get a call complaining about burndown failures behind a ground rig applying proper rates with 10 gallons of water when the wind is blowing less than 10 miles per hour.

I have had several questions about using higher than 8 ounce rates of dicamba. We have sprayed a lot of these in plots and it should be no surprise that the 10-ounce rate and the 12-ounce rate of dicamba provide quicker and more consistent horseweed control.

The plant-back interval is 21 days following rainfall per 8-ounce rate. Therefore, if higher rates are used, the plant-back interval is lengthened. By extrapolation, it can be determined that the plant-back interval for 10 ounces is 29 days after rainfall.

There will be at least one new herbicide in the burndown market this year called Sharpen. It and some of our research findings will be covered in future articles.